Specially designed mechanical switching devices are known for making, carrying, and breaking electrical circuits under normal conditions as well as performing in a special way under abnormal conditions. These are common devices to protect a circuit against excess current flow, a useful example of which is called a fuse.
Thermal fuses useful in electrical or electronic applications are known. U.S. Pat. No. 4,267,542 describes a device for a thermal fuse for use with an electrical apparatus in which easy access to the thermal fuse element (not specifically described) is allowed. U.S. Pat. No. 4,313,047 describes a fuse element which provides thermostatic control and thermal fuse overtemperature protection for an electrical heating device. U.S. Pat. No. 4,581,674 describes a thermal fuse element comprising an alloy having a eutectic composition at a predetermined threshold. Upon exposure to excessive heat, the alloy melts and the circuit opens. U.S. Pat. No. 4,757,423 relates to a fuse for an electronic component which incorporates a pad of a fusible material, preferably comprised of metal coated polymeric particles. Upon overheating, the metal melts and is dispersed within the polymer.
Midwest Components, Inc., Product Data Sheet (1987) and Raychem Product Data Sheets (5/88, 11/87, 10/89, 6/90, and 4/91) disclose PolySwitch.TM. Products for reversible circuit breaking applications. The articles contain a homogeneous mixture of polyolefin and carbon and have an electrical resistance which increases with temperature or overcurrent.
Expanded polytetrafluoroethylene-containing articles are known to provide thermal insulation. Related U.S. Pat. Nos. 3,953,566, 3,962,153, 4,096,227, and 4,187,390 teach a porous product comprising expanded, amorphous locked PTFE which can be laminated and impregnated to provide shaped articles. The more highly expanded materials of that invention are disclosed to be useful, for example, as thermal insulators and shaped articles.
PTFE fibrillated matrices are known. The background art teaches several formulations for blending an aqueous PTFE dispersion with various additives and/or adjuvants designed for specific purposes. For example, U.S. Pat. No. 4,990,544 teaches a gasket comprising a fibrillated PTFE resin and dispersed therein a fine inorganic powder. U.S. Pat. No. 4,985,296 teaches an expanded, porous PTFE film containing filler material which is purposely compressed to provide thin films where space reduction is desirable.
U.S. Pat. Nos. 4,971,736, 4,906,378, and 4,810,381 disclose a chromatographic sheetlike article and method of preparing a composite chromatographic sheetlike article comprising a PTFE fibril matrix and nonswellable sorptive hydrophobic particles enmeshed in the matrix. References cited in these patents relate to other PTFE matrices containing particulates, including U.S. Pat. Nos. 4,153,661, 4,373,519, 4,460,642, and 4,565,663.
It is known that metals can be incorporated in fibrillated PTFE, as in, for example, U.S. Pat. No. 4,153,661. U.S. Pat. No. 4,923,737 discloses a method for a "metal cloth" prepared from fibrillated PTFE containing metal or other particles entrapped in the fibrils.
A composition comprising fibrillated PTFE in combination with a polyamide has been disclosed to provide articles by extrusion blowmolding as in U.S. Pat. No. 4,966,941, and with molybdenum disulfide and optionally an elastomer to provide articles with increased durability as in U.S. Pat. No. 4,962,136.
U.S. Pat. No. 4,945,125 teaches a process of producing a fibrillated semi-interpenetrating polymer network of PTFE and silicone elastomer. U.S. Pat. No. 4,914,156 describes a blow moldable composition comprising a polyether, an epoxide polymer, a source of catalytic cations, and a fibrillatable PTFE. U.S. Pat. No. 4,902,747 discloses a blow moldable polyarylate composition containing fibrillatable PTFE.
Vermicular expanded graphite has been incorporated into PTFE. U.S. Pat. Nos. 4,265,952 and 4,199,628 relate to a vermicular expanded graphite composite blended with a corrosion resistant resin such as PTFE with improved impermeability to corrosive fluids at high temperatures.
Conductive compositions comprising a polymeric binder system having dispersed therein electrically conductive particles and deformable non-conductive spherical domains have been disclosed, for example, in U.S. Pat. No. 4,098,945. Similar compositions have been disclosed to be useful as fuses in R. Woolnough, Electronic Engineering Times, Dec. 2, 1991, p 39.
U.S. Pat. No. 4,483,889 teaches a method for making a foam composite material comprising a fibrous matrix, expandable polymeric microspheres, and a formaldehyde-type resin.